Substrates that are designed to isolate particulate matter or to form a barrier from fluids, gasses, light, and solids are well known in the art. Included in such substrates are barrier films that are designed to prevent the transmission of a particular fluid across the barrier. Examples of fluids for which barrier films are utilized include oxygen and water. Typically, the barrier film is located between a source of the fluid and an object or region to be protected from the fluid. Another class of such substrates is membrane systems. Typically these membrane systems utilize a barrier that is capable of separating mixtures of fluids or separating solids or particulates from a fluid. One class of membranes separate a mixture of different gaseous materials into components, for instance oxygen from nitrogen or carbon dioxide from an air stream. Another class of membrane systems is designed to separate particulate matter or entrained or dissolved chemical species from a liquid. One example includes a reverse osmosis membrane designed to remote salt from brackish or salt water. Another example includes an ultrafiltration membrane which can remove higher molecular weight organic compounds from a liquid. Another class of substrates is wall flow filters. Wall flow filters are typically ceramic based and separate particulates from fluids based on size. A common design of a wall flow filter comprises a shaped ceramic article with flow passages that extend through the article in one direction. Wall flow filters have in one direction a series of walls that define flow passages generally arranged such that the walls and flow passages are disposed parallel to one another and the walls and passages extend through the shaped ceramic article. In the direction perpendicular to the walls and flow passages the wall flow filters generally demonstrate a consistent cross-sectional shape. The cross sectional shape can be any shape which is suitable for the intended use of the wall flow filter. The cross sectional shape can be circular, oval, square, rectangular, polyhedral or a shape defined by an assembly of square, rectangular or polyhedral shaped parts. In some embodiments, the wall flow filter exhibits two faces at each end having the desired cross-sectional shape and the flow passages are perpendicular to the faces of the filter and extend from one end or face to the other end or face. Often, this arrangement is referred to as a honeycomb design because each end of the filter looks similar to a honeycomb. In a preferred embodiment the wall flow filter has a cylindrical shape having a circular or oval cross sectional shape. In another preferred embodiment, the wall flow filters comprise a plurality of individually formed parts that are assembled together to form a desired cross section in the direction perpendicular to the direction of the flow passages. In this last instance the cross section of the wall flow filter comprises an assembly of the cross section of the parts used to prepare the wall flow filter and can be engineered to have any desired shape. Wall flow filters often are arranged having a plurality of walls defining a plurality of flow passages. In wall flow filters at one end every other flow passage is plugged such that the fluids cannot pass through the end of the plugged flow passage. At the other end the remaining passages are plugged in a similar manner. The arrangement results in a structure such that each flow passage is open at one end and plugged at the opposite end. Each flow passage is surrounded by passages that are plugged at the opposite end from which it is plugged. In order to separate particulates from a fluid stream, the fluid stream is introduced through one end of the filter into the flow passages in that end. Because the other ends of the flow passages are plugged the fluid can only exit the filter through the porous walls of the flow passage and into flow passages adjacent to the flow passage into which the fluid is introduced. The flow passages into which the fluid passes are open at the opposite end from fluid introduction. Typically, a pressure differential is maintained between the flow passage into which the fluid is initially introduced and the flow passages adjacent thereto to drive the fluid through the flow passage walls. The particulate matter contained in the fluid which is of a size greater than the pores in the walls of the flow passage is retained on the wall of the flow passage into which the fluid is introduced. The fluid flowing out of the opposite end of the filter is substantially free of particulates of a size greater than the pores found in the walls of the flow passages. In a preferred embodiment the manufacture of the wall flow filters is adapted to produce walls with relatively uniform pores to facilitate the desired separation. The design and manufacture of ceramic wall flow filters is well known in the art and not subject of this invention. Substrates that can be used as films or membranes which are substantially flat and the fluids to be separated or purified are contacted with one side of the film and either prevented from passing through the film or the desired fluid is passed through the film and collected on the opposite side and the undesired material is retained on the original surface of contact. In other embodiments, the substrate is arranged in another way in a device having an inlet for feeding the fluid to be separated and an exit for recovering the purified fluid. This exit may be remote from the actual separating species. Examples of structures of this type are wall flow filters, hollow fiber membranes and spiral wound membranes systems.
In the devices in which these barrier films, membranes or filters are incorporated defects in fabrication can allow undesired fluids or particulate matter to flow into the recovered fluid. Also if the barrier films, membranes or filters have defects undesired fluids or particulate matter can pass through the barrier films, membranes or filters. Such defects render them unsuitable for use. Thus it is desired to identify defective barrier films, membranes or filters and systems containing them which due to defects cannot effectively utilize the barrier properties or perform the separation required. Methods and apparatus for identifying such defects in wall flow filters are known in the art, see Kato, US 2009/0051909; Gargano et al US 2007/0022724; Gargano et al US 2007/0238191; Hijikata et al U.S. Pat. No. 5,102,434; and Zoeller, III U.S. Pat. No. 7,520,918; all incorporated herein by reference. All of these disclosed systems and methods require the use of highly directional light sources, lasers, wherein a very thin sheet of light is used to locate the particulate matter exiting the wall flow filters. These methods require that the thin sheet of light be located at a distance from the surface of the wall flow filter.
There is still a need for systems and methods of identifying and locating defects in barrier films, membranes, filters and systems containing the barrier films, membranes or filters. Systems and methods which operate in a non-destructive fashion, which identify and locate the defects in a timely manner as part of the manufacturing process and which can identify the location of the defect in the barrier films, membranes or filters or at the exit from the system containing the barrier films, membranes or filters are desired.